Papers by Author: Sang Kwon Lee

Abstract: The Location Template Matching (LTM) method is known to be simple and straightforward to use for source localization problem. We have found that the method is not as simple as one might think, but there are many practical aspects that must be carefully considered. The most important parameter is the spatial resolution of the pre-defined reference points. The other parameters are the sampling rate, the time alignment between the signals, the frequency bandwidth of a signal, and the length of data. It is also shown that the accuracy of the LTM method can be improved if a couple of more sensors are used. This paper discusses these practical considerations in detail for the purpose of providing a proper guidance of using the LTM method.

Abstract: Tire noise is classified by pattern noise and road noise in a vehicle. Especially pattern noise has impulsive characteristics since it is generated by impacting of tire’s block on the road. Therefore, a special signal process is needed other than traditional Fourier Transform, because the characteristic of signal is varying with time. On the other hand, the pattern noise is a kind of non-stationary signal and is related to the impulsive train of pitch sequence of a block. In this paper, Wavelet Transform is applied to verify the impulse signal caused by impact of block and groove and to verify the relationship between the pattern noise and the train of pitch sequence.

Abstract: The aim of this paper is to present the method of identifying the impact location on the plate. This basic research has the purpose to achieve the human-interaction technology based on the signal processing, piezoelectric materials, and wave propagation. The work concerning the location identification of a single impact on the plate has been previously studied by means of the time frequency analysis applied to accelerometer signals. In this paper, a novel approach for the location identification of the discrete impact is presented to investigate the feasibility of applying to a human-interaction system. For the identification of the impact location, the major axisymmetry mode of the Lamb wave on the plate is used for this work.

Abstract: A new continuum damage theory (CDT) has been proposed by Lee et al. (1997) based on the SEEP. The CDT has the apparent advantage over the other related theories because the complete constitutive law can be readily derived by simply replacing the virgin elastic stiffness with the effective orthotropic elastic stiffness obtained by using the proposed continuum damage theory. In this paper, the CDT is evaluated by comparing the mode shapes and natural frequencies of a square plate containing a small line-through crack with those of the same plate with a damaged site replaced with the effective orthotropic elastic stiffness computed by using the CDT.

Abstract: Impulsive vibration signals in gearbox are often associated with faults, which lead to due to irregular impacting. Thus these impulsive vibration signals can be used as indicators of machinery faults. However it is often difficult to make objective measurement of impulsive signals because of background noise signals. In order to ease the measurement of impulsive signal embedded in background noise, we enhance the impulsive signals using adaptive signal processing and then analyze them in time and frequency domain by using time-frequency representation. This technique is applied to the diagnosis of faults within laboratory gearbox.

Abstract: Impulsive sound and vibration signals in gear system are often associated with their faults. Thus these impulsive sound and vibration signals can be used as indicators in condition monitoring of gear system. The traditional continuous wavelet transform has been used for detection of impulsive signals. However, it is often difficult for the continuous wavelet transform to identify spikes at high frequency and meshing frequencies at low frequency simultaneously since the continuous wavelet transform is to apply the linear scaling (a-dilation) to the mother wavelet. In this paper, the spike wavelet transform is developed to extract these impulsive sound and vibration signals. Since the spike wavelet transform is to apply the non-linear scaling, it has better time resolution at high frequency and frequency resolution at low frequency than that of the continuous wavelet transform respectively. The spike wavelet transform can be, therefore, used to detect fault position clearly without the loss of information for the damage of a gear system. The spike wavelet transform is successfully is applied to detection of the gear fault with tip breakage.

Abstract: In an automotive engine, faults induce impulsive vibrations and thereby degrade engine performance, making it important for an automotive engineer to detect and analyze impulsive vibration signals for fault diagnosis. However, detecting and identifying impulsive signals is often difficult because of interfering signals such as those due to engine firing, harmonics of crankshaft speed and broadband noise components. These interferences hinder early fault detection. To overcome this difficulty we present a two-stage ALEF (Adaptive Line Enhancer Filter) that is capable of enhancing impulsive signals embedded in background noise. This method is used to pre-process signals prior to time-frequency analysis via higher order methods such as the combined higher order time-frequency.

Abstract: Impulsive vibration generated by localized gear damage can be used as an indicator for damage detection. Local damage induces an abrupt increase of the amplitude and phase lag of the impulsive vibration signal measured on the gearbox. Relatively large damage like “tip breakage” can be easily detected by the amplitude map of CWT (continuous wavelet transform) for the impulsive vibration signal measured on the gearbox. However, minor damage like “initial pitting” cannot be detected with the amplitude map. To overcome this problem, in this paper we take into account the phase map for a damage signal. The zoomed phase map of CWT is successfully applied to the detection of minor gearbox damage.

Abstract: This paper presents experimental results of source identification for a non-minimum phase system. Generally, a causal linear system may be described by matrix form. The inverse problem is considered as a matrix inversion. Direct inverse method cannot be applied for a non-minimum phase system, because the system has ill-conditioning. Therefore, in this study the SVD inverse technique is introduced to execute an effective inversion. In a non-minimum phase system, its system matrix may be singular or near-singular and has very small singular values. These very small singular values have information about a phase of the system and ill-conditioning. Using this property we could solve the ill-conditioned problem of the system and then verify it for the practical system (cantilever beam). The experimental results show that the SVD inverse technique works well for a non-minimum phase system. This inverse technique can be applied to the estimation of the magnitude of impact force, which becomes often a cause of damage to a mechanical system.

Abstract: In mechanical structures, the impact force is related to the structural damage. To identify the location where impact force occurs, the triangle method has long been used. This method requires three acceleration signals or strain signals to be measured on the mechanical structure. Time delay among these signals is useful information to estimate the location of the impact force. It is very difficult to estimate time delay by using the raw data of three signals because the propagation wave of the structure is a dispersive wave. Therefore, three signals should be analyzed in the time and frequency domain in order to estimate the time delay at each frequency. For the time-frequency analysis of highly non-stationary signals like impulse signals, time-frequency methods or time scale methods have been used. These methods use the first or second order statistical characteristics of the signal. This paper outlines the higher order Wigner method to obtain time and frequency information of a signal. Since it uses the high order statistics of signals, this method is useful for identifying the impact signal embedded in the background. It has a better time-frequency resolution for a non-linear signal than other time-frequency and time scale methods. This method can be applied to estimate the location of an impact force, which becomes a cause of damage of mechanical plants. Finally, in order to prove this method, experimental work was conducted on an aluminum plate in the laboratory.